Drill bits used to drill wellbores through earth formations typically have a nominal diameter, that is, the diameter that will be created when the drillbit is rotated and impressed axially onto the formation. Frequently it is desirable to enlarge the diameter of the borehole beyond the nominal diameter of the drillbit for several purposes. It is well known that in the process of drilling a deep well for oil and gas production; under reaming of a borehole, it is necessary to treat the borehole wall so as to prevent reduced predetermined diameter of the borehole and also to enlarge the diameter of the borehole drilled by the main bit in front of the drillstring.
Numerous attempts have been made to provide proper tools for enlarging a borehole particularly in the oil wells. One of the known prior art attempts is to use specialized drill bits, known as bi-centre bits. Bi-centre bits have been developed to create boreholes with drilled diameter greater than the diameter of the opening through which such bits will pass when they are not rotated. U.S. Pat. Nos. 6,340,064; 6,394,200; 5,992,548; 5,678,644 are examples. Rather than employing a one-piece drilling structure, such as an eccentric bit or a bi-centre bit to enlarge a borehole, it is known that reamers including reamer wings typically are assembled to a drillstring at an axial position behind the drillbit. Underreamers have basically two operating states, a closed or collapsed state which enables them to pass through the narrowest section of the borehole and an open or partly expanded state where one or more arms with cutters extend radially outward from the rotational centre of the tool body. When the reamer rotates, the cutting elements on the extended arms enlarge the boreholes. U.S. Pat. Nos. 4,589,504; 5,368,114; 5,060,738; 4,431,065 are examples of hole-openers with solid expandable arms.
It is also known in the prior art that roller side cutters are used instead of the solid cutter blades to ream and enlarge a borehole. Examples of roller side cutters are found in the patent documents U.S. Pat. Nos. 2,122,763; 2,172,762; 2,189,033; 2,189,037; 2,199,693; 2,260,366; 2,306,492; 3,306,381; 3,627,068; 3,907,048; 4,182,425; 4,398,610; 4,036,314. U.S. Pat. Nos. 6,378,632; and 3,917,011 are special cases using rotary roller cones to increase the borehole diameter.
Moreover it is known from US 2004/0134687 A1 to use an expandable reamer apparatus and methods for reaming a borehole, wherein a laterally movable blade carried by a tubular body may be selectively positioned at an inward position and an expanded position. The movable blade, held inwardly by blade-biasing elements, may be forced outwardly by drilling fluid selectively allowed to communicate therewith by way of an actuation sleeve disposed within the tubular body.
The publication U.S. Pat. No. 6,378,632 B1 discloses an underreamer that opens a borehole below a restriction that is larger than the restriction itself. The underreamer includes cutters which engage the formation by traversing outward and upward. The force pushing the cutters to the extended position is supplied by a piston built into each cutter support. Pressure acting on these pistons comes from the pressure differential between the annulus and drill string during circulation of drilling fluid.
GB 1208127 relates to well reamers and more particularly to well reamers having a single-roller-cutter used for enlarging well bores.
Other examples of reamers can be found in WO 02/064939 A1, U.S. Pat. No. 6,378,632 B1 and US 2009/0294173 A1.
Enlarging the borehole diameter with cutting blades or with passive rollers is associated with increased drillstring torque. In addition, due to operational limitation such as vibration, the enlarged diameter is limited when using passive cutters. Use of the active rotary cutters is favourable with respect to the torque and will also enable to drill larger borehole than what is possible today.
According to the present invention it is an object to provide a tool which overcomes the drawbacks of increased drillstring torque and vibration which occur when using prior art devices.